Numerous electromagnetic mass launchers have been developed for accelerating a projectile to high velocity. In a typical prior art electromagnetic launcher, a travelling magnetic wave is generated along a cylindrical barrel while an electric current is simultaneously generated on the surface of or within the armature. As the magnetic wave propagates along the barrel, rail or track, the force resulting from the electromagnetic pulse and the induced armature current accelerates the projectile. To energize the armature of the projectile, current is initially generated in the launcher, and then induced in the armature. In contrast, in some of the earliest launchers, current was conducted to the armature by means of sliding contacts connecting a portion of the launcher to the armature. And yet in other launchers, current was conducted to the armature by means of electromagnetic arcing between launcher and the armature.
Recently, launchers have been devised and operated using induction rather than sliding contacts or arcs to produce the electric current in the armature of the projectile. More particularly, two basic types of such induction launchers have been developed.
In the first type of induction launcher, called a "pulsed-induction" launcher, magnetic field pulses are sequentially generated by successive barrel coils with the pulses being synchronized with the aft portion of the projectile such that currents are induced on the aft surface of the projectile. An increase of acceleration is achieved by increasing the magnetic field strength of the applied magnetic pulses. However, the current induced on the aft surface of the projectile decays because of the resistive nature of the armature and the magnetic field then penetrates forward. This penetration severely degrades the efficient transfer of energy from the coils to the projectile and reduces acceleration. To overcome the performance loss, the direction of the applied magnetic field can be reversed from stage to stage. However, this leads to concentration of current on the armature surface and significant ohmic heating and armature ablation. The temporal acceleration pulse is also highly peaked and choppy. Because the launcher length is determined by average acceleration and the projectile design is dependent upon the peak acceleration, an excessive launcher length is required for a given projectile design.
In the second type of induction launcher, a sinusoidal magnetic field is generated within the barrel. Sinusoidal travelling wave launchers do not use pulsed induction methods; rather, currents in the barrel drive coils are sinusoidally oscillated according to a phased time sequence and with the frequency of oscillation increasing as the speed of the projectile increases. An increase of acceleration is achieved by increasing the strength of the sinusoid applied magnetic field. The sinusoidal magnetic field is timed to travel slightly faster than the projectile such that, as the projectile lags the applied magnetic field, the surface of the projectile experiences a time rate of change in magnetic field which induces a current on that surface. The difference in speed between the travelling magnetic wave and the projectile is commonly referred to as the "slip speed".
In the sinusoidal continuous wave launchers, the current penetrates the armature more deeply, thus, ohmic heating is reduced in the armature. However, the barrel coils are energized for long periods of time leading to coil heating and inefficient operation because energy dissipated by ohmic heating cannot be transferred to the projectile. Moreover, current concentration at the front and rear of a projectile increase armature heating in those regions and, in the front of the armature, cause a retarding force.
It is thus an object of the invention to minimize ohmic heating in the armature of a cylindrical launcher. The feature of the inventive method that achieves this object is to select firing times of the coils so that the induced current wave in the armature has a low but non-zero frequency.
It is a further object of the invention to provide for smooth acceleration of the projectile through the launcher. One feature of the inventive method that achieves this result is the synchronization of firing the magnetic pulses from the coil stages so that no pulses are applied beyond the forward surface of the projectile to generate any retarding forces. Another is the relatively long length of the armature compared to the coil.
It is a further object of the invention to provide for minimal damage to the payload. In achieving the smooth acceleration as stated above, no forces are applied to the front of the projectile, so that a payload positioned on the front end of the projectile does not experience damaging forces, heating, and induced currents and voltages in payload electronic components.
It is yet another object of the invention to further maximize performance. The invention achieves this object by minimizing ohmic heating in the coils and in the armature.
These and other objects of the invention will become apparent upon a reading of the summary and the detailed description of the invention.